Correlation Measurements for Process Quality Analysis

ApplicationsDetection of Periodicities

Prediction of Signals in ‘Noise’

Measurement of Time Delays

Location of Disturbance Sources

DepartmentsResearch and Development

Production Testing

Engineering Test

Quality Assurance

Description

Paper is the culmination of a long series of process

stages that are all prone to variations due to raw

materials, time and operator set up. While there are

several properties that are used to define the quality

of paper, its weight (basis weight) is the most

fundamental and reducing the variation in basis

weight has been pursued with the greatest vigor of

any single property. Low frequency, machine

direction basis weight variation is usually traceable to

problems upstream in the machine, in the stock

approach system. The problems generally stem from

either pulsation in the stock system or from an

inability to maintain a uniform consistency in the

stock.

Problem

Troubleshooting an MD weight variation requires the

ability to simultaneously acquire and analyze a

variety of signals such as on-machine basis weight,

thin stock consistency, stock approach system

pressure (at several locations), vibration and drive

motor parameters such as speed, etc. A complex array

of time and frequency domain measurements is then

required to properly characterize and solve the

problem. Users often attempt to perform only

frequency domain measurements to analyze the

nature and sources of variation in the system with

coherence function measurements to provide a

measure of causality between different signals.

However, in most process environments, it is essential

that a degree of similarity as well as the direction of

propagation along with lag time is available when

comparing different signals. Correlation

measurements are the only answer to problems like

these.

Solution

The SignalCalc Mobilyzer equipped with 8 to 16

channels is the ideal platform for fast and flexible

in-mill data acquisition, analysis and reporting. The

Application Note: DP-AN05-001

Poor tooth contact

Windows based Dynamic Signal Analyzer software

offers Correlation measurements as a standard

feature. Users can view the unfiltered and filtered

time domain data for all channels and analyze the

average cross correlation measurements between

the various signals. All this can be accomplished

while recording raw data simultaneously at either

the measurement sample rate or a different sample

rate, specified by the user.

Implementation

The measurements begin with the setup of

multiple sensors coupled to the Abacus front-end.

The high input impedance, differential inputs of the

acquisition module allow direct connection to the

on-machine basis weight scanner that is generally

operated in single-point mode for the correlation

measurements. Pressure sensors on both sides of

the pumps, screens, de-aeration devices and the

headbox along with thin and thick stock

consistency meters provide the other essential

measurements for the analysis. By adjusting the

length of time blocks and with the use of special

filters that are user defined, the magnitude and

source of the basis weight variations may be

identified.

Key System Features

Intuitive and Flexible Graphical User InterfaceEU Offset Calculation for Signals with DC contentBi-Directional, Normalized Cross CorrelationRemoval of Mean Value before CorrelationUser Definable Low Pass, High Pass, Band Pass and Band Reject FiltersUnlimited Display LayoutsThroughput Recording to Local Bus Disk

-100 -50 0 50 -0.100

-0.050

0

0.050

0.100

0.150

0.200

0.250

0.300

sec

Real

Reel BW Scanner, Hdbx TS

R1, 7Peak #1 X: -29.3457 Y: 0.2959

20 40 60 80 100 120 140 2.260

2.270

2.280

2.290

2.300

2.310

2.320

2.330

2.340

2.350

2.360

sec

Real,

psi

Hdbx TS

PkPk: 0.0852Mean: 2.3071StdDev: 0.0142

X7 X: 0 Y: 2.3001X7 X: 150.0000 Y: 2.2885 dX: 150.0000 dY: 0.0115

40 60 80 100 120 140 160 51.500

52.000

52.500

53.000

53.500

54.000

54.500

55.000

sec

Real,

lbs/3

300ft

Reel BW Scanner

PkPk: 2.3513Mean: 53.3409StdDev: 0.4172

X1 X: 29.3457 Y: 53.3406X1 X: 179.3457 Y: 52.9272 dX: 150.0000 dY: 0.4135

Hardware

DP700-30-A1Network Peripheral

DP700-30-16CDSPCentric Signal ProcessingHardware – 16 Channels

DP730-23Throughput to Disk and PlaybackAnalysis

Part Number and Description Qty

DP730-10Windows Based Dynamic Signal Analyzer Software

1

1

1

1

Software